1. Technical Field of the Invention
The present invention relates to road motor vehicles with series hybrid motorization.
2. The Related Art
In vehicles of this type, an engine drives an electric alternator which converts the mechanical energy available at the shaft of the engine into electrical energy. This electrical energy supplies one or more electric traction motors, mechanically connected to the driving wheels of the vehicle. The engine is not mechanically connected to the wheels, unlike vehicles with parallel hybrid motorization, in which an engine and an electric motor are both mechanically connected to the driving wheels, the torques which they deliver being able to be added together to drive the driving wheels.
Drivers of motor vehicles with conventional motorization (the expression xe2x80x9cconventional motorizationxe2x80x9d is intended here to mean an engine and a gearbox, manual or automatic) are familiar with the acceleration and braking controls which are well known in the prior art. These controls have attained a high degree of progressivity and reactivity. It is desirable for a hybrid vehicle to be able to be driven substantially in the same way as a conventional motorization vehicle, so as not to confuse a driver used to conventional motorization. It is therefore a question of being able to transform the actions of a driver on the accelerator pedal, and more generally on the accelerator and brake pedals, into judicious actions on the regulation of the drivetrain starting from the engine and ending at one or more electric traction motors.
It is well known to install an electric storage battery as a buffer between the alternator and the electric motor, in particular where it is desired to be able to drive the vehicle in purely electric mode, with the engine switched off. In this case, the regulation of the engine and the control (in the electrical sense of the term) of the electric traction motor can be independent. There is no particular problem with controlling the torque of an electric motor, which draws its energy from an electric storage battery, with all the desired progressivity and reactivity to the accelerator pedal at the disposal of the driver.
However, the use of an electric storage battery presents various problems. Such a battery has a high mass compared with the amount of electrical energy stored. This considerably increases the mass of a vehicle and is a source of waste during acceleration, not to mention the problems with the dynamic behavior of a vehicle, which are all the more crucial if the vehicle is heavy. Moreover, such batteries present maintenance problems and environmental problems on account of the numerous pollutants which they contain and which are difficult to recycle.
The object of the present invention is therefore to design a series hybrid drivetrain which can eventually operate without an electric storage battery, while affording the vehicle driver a very progressive and very reactive accelerator control. Where there is no electrical energy store (battery), it is necessary to be able to produce just the electrical energy necessary to cover the demand. The problem which arises is therefore to obtain the torque required at the electric traction motor while avoiding stalling or racing of the engine, and to do this by means of a control whose progressivity and reactivity are as close as possible to those of an accelerator pedal of a conventional thermal vehicle.
The invention proposes a drivetrain for a series hybrid vehicle comprising:
an engine driving an alternator, the engine having a power available at the output shaft of the engine;
at least one electric traction motor connected to an alternator by an electric line and an inverter, the inverter allowing the electric traction motor to be operated at a desired torque, the electric line allowing the transfer of an electric traction power;
an accelerator control CA at the disposal of the vehicle driver;
an actuator acting on the engine;
a control device controlling the position of the actuator as a function of the position of the accelerator control, as far as a limitation at least when the desired torque reaches the maximum torque of the electric traction motor; and
a unit for controlling the propulsion torque of the vehicle, allowing the desired torque to be continuously calculated as a function of the vehicle speed and as a function of a control power evaluating the power available at the output shaft of the engine as a function of the actual speed of the engine.
The engine may be of any type which can be controlled by a variable control such as an accelerator pedal. For example, diesel engines are known in which the control acts on the amount of fuel injected into the combustion chamber. Petrol engines are also known in which the control acts on the amount of air admitted into the combustion chamber. These examples are not limiting. The fuel injection control of a diesel engine, as well as the throttle control of a petrol engine, are an xe2x80x9cactuator PPxe2x80x9d in the context of the present invention.
As for the electric traction motor, there may be one or more of them, for example one per vehicle wheel. If there are a plurality of electric traction motors, xe2x80x9cdesired torque Cxe2x80x9d is to be understood as a combined overall torque for all the motors, the present invention not being concerned with the question of the distribution of the torque between the motors. As for the type of electric motor, this is a motor whose torque delivered to the rotary shaft can be controlled in terms of amplitude and sign. For example, a Permanent Magnet Synchronous Motor (PMSM) is used, with flux concentration.
The alternator used is, for example, an electric machine of the same type as that proposed for the electric traction motor. The alternator converts the mechanical energy available at the output shaft of the engine into electrical energy consumed by the electric load connected to the electric line fed by the alternator (for this disclosure, the electric load is mainly the electric traction motor(s)).
The control power P comes from an evaluation of the available power Pthd originating from the engine. The control power P does not matching exactly the actual available power Pthd. Various curves for control power as a function of the actual speed R will be given below, which curves are superimposed on the real curves for the available power Pthd for various positions of the actuator PP. Since the control power P comes from an approximation of reality to a greater or lesser extent, the operating point of the engine giving a particular power as a function of the actual engine speed R does not necessarily correspond to an equilibrium between the actually available power Pthd and the power consumed in electrical form by the loads connected to the alternator. The calculation of the desired torque C is carried out by successive iterations. As long as there is a difference between the power delivered by the engine and the power consumed in electrical form, the speed of the engine changes, and therefore the evaluation of the actually available power Pthd changes. The next iteration of the calculation of the desired torque C by the control unit determines a new value for the desired torque C, thereby bringing about a variation of the electric power consumed by the loads connected to the alternator, which tends to reduce the difference, and so on until the difference is eliminated.
It will be explained below how a curve for control power (called more simply xe2x80x9ccontrol curvexe2x80x9d below) as a function of the actual speed R can be chosen so that the electric power consumed by the loads connected to the alternator tends to balance the available power.
It should be pointed out that, according to one aspect of the present invention, the accelerator control CA at the disposal of the driver does not act directly on the desired torque C of the electric traction motor. It acts on the engine. The desired torque C of the electric traction motor is automatically adapted to the actual operation of the engine. Thus, the accelerator control CA acts directly on the actuator PP. The actuator PP also takes account of the torque absorbed by the electric traction motor, since this is necessarily limited, otherwise the electric motor would be destroyed in the event of excessively strong current. This is because, at low vehicle displacement speed, saturation (maximum current admissible by the electric motor and by the inverter) is reached for low power levels. To avoid racing of the engine, it is advisable to limit the demand for acceleration coming from the driver. It is only below saturation of the electric motor that the control of the actuator PP may be linked, for example proportionally, to the accelerator control CA on which the driver acts directly.
Of course, to the extent that electrical energy is available on the electric line mentioned, it may be envisaged to connect various electric loads other than the electric traction motor(s). In certain cases, if only in a transitory phase, these other electric loads may absorb considerable electric power, sometimes more than the electric traction motor, and even more than the available power. In this case, it is advisable to make various adjustments to the regulation principle explained above, which will be dealt with hereinbelow.